Sound probleme with au-117

Hy everybody.I've a old sansui au-117(year 1978) which have problem of sound in the right chanel:there is sound but it's poor and like with overdrive, the left is perfect.I have change the place of the transistors sb507 and sd313 with no results.Can it be the 2 Elna caps(4700µf_35v_85°c)?The electronics parts seems to be.Please give me some ways to go.Excuse my language caus i'm french guy.By!!